This application relates generally to optical imaging. More specifically, this application relates to optical topographic imaging.
There are a variety of applications in which it is desirable to study an object in order to analyze its topographical features. One particular example is fingerprint imaging, in which the topographical pattern of ridges is commonly used in identification applications, either to provide a mechanism for identifying a person from fingerprints or to verify a purported identity of a person based on that person's fingerprints.
Most fingerprint-collection methods that are currently used rely on measuring characteristics of the skin at or very near the surface of a finger. In particular, optical fingerprint readers typically rely on the presence or absence of a difference in the index of refraction between a sensor platen and skin tissue placed on it. When an air-filled valley of the fingerprint is above a particular location of the platen, total internal reflectance (“TIR”) occurs in the platen because of the air-platen index difference. Alternatively, if skin of the proper index of refraction is in optical contact with the platen, the TIR at this location is frustrated, allowing light to traverse the platen-skin interface. A map of the differences in TIR across the region where the finger is touching the platen forms the basis for a conventional optical fingerprint reading. There are a number of optical arrangements used to detect this variation of the optical interface in both bright-field and dark-field optical arrangements. Commonly, a single quasimonochromatic beam of light is used to perform this TIR-based measurement.
There also exist other optical fingerprint sensors that do not rely on this TIR-based measurement. In most cases, such sensors rely on some arrangement of quasimonochromatic light to illuminate the front, sides, or back of a fingertip, causing the light to diffuse through the skin. The fingerprint image is formed from differences in light transmission across the skin-platen boundary for the ridge and valleys. The differences in optical transmission are due to changes in the Fresnel reflection characteristics that result from the presence or absence of any intermediate air gap in the valleys.
Another example of in which imaging is used to analyze topographical features are in the deployment of machine-readable symbols that are incorporated directly onto parts being marked as part of supply-chain monitoring systems. The symbols are used to improve supply-chain management by marking parts with unique symbols having topographic features formed by such techniques as laser etching, chemical etching, dot peening, casting, machining, and other operations. The ability to image, and thereby read, the unique symbols is fundamental to the operation of monitoring the transfer of parts and materials through supply chains. Such symbols are referred to herein as “machined barcodes,” but such terminology is not intended to limit the manner in which the symbols are formed.